Problem with Double Wye Capacitor Unbalance Protection
Problem with Double Wye Capacitor Unbalance Protection
(OP)
Dear All,
I am talking about Double Wye ungrounded Capacitor Unbalance protection using Neutral CT and neutral VT as explained in IEEE C37.99-2000 section 8.2.6.2.
i have 13.8KV capacitor bank double way ungrounded and both unbalance protection approches are used:
CT approach: CT will be connected to 51N relay in order to detect any fuse blown. the problem of this apprach in that no current will flow in the neutral in case of identical fuse blown in both legs of the same phase.
VT apprach: VT wil be connected to 59N relay to detect any unbalance. this apprach overcomes the 51N limitation.
the prblem i am facing is that 59N relay operated many time for other 13.8KV feeders faults while the 51N is stable.
any explanation please?
thank you,
I am talking about Double Wye ungrounded Capacitor Unbalance protection using Neutral CT and neutral VT as explained in IEEE C37.99-2000 section 8.2.6.2.
i have 13.8KV capacitor bank double way ungrounded and both unbalance protection approches are used:
CT approach: CT will be connected to 51N relay in order to detect any fuse blown. the problem of this apprach in that no current will flow in the neutral in case of identical fuse blown in both legs of the same phase.
VT apprach: VT wil be connected to 59N relay to detect any unbalance. this apprach overcomes the 51N limitation.
the prblem i am facing is that 59N relay operated many time for other 13.8KV feeders faults while the 51N is stable.
any explanation please?
thank you,






RE: Problem with Double Wye Capacitor Unbalance Protection
RE: Problem with Double Wye Capacitor Unbalance Protection
yes, the VT is connected from the neutral point to ground. the problem is what you sid. the netral point is shift due to any unbalance in the system which causes the relay 59N to operate.
So, what is the correct application of this type of protection?
RE: Problem with Double Wye Capacitor Unbalance Protection
I believe there are other methods shown in that standard which involving multiple PT's to account for the phase inbalance.
RE: Problem with Double Wye Capacitor Unbalance Protection
RE: Problem with Double Wye Capacitor Unbalance Protection
Regards
Marmite
RE: Problem with Double Wye Capacitor Unbalance Protection
I agree with you.
Usually we set the unbalance protection based on manufacturer unbalance calaculation which shows how much voltage will increase in other capacitor units.
the setting of the unbalance protection is set based on that calculation.
So, the objective is not to dtect the blown fuse but to protect the other capacitor units.
thanks
RE: Problem with Double Wye Capacitor Unbalance Protection
I think, per the formula in C37.99, that your scheme should detect open fuse conditions. Just make sure that the 59N relay is restrained for third-harmonic conditions.
RE: Problem with Double Wye Capacitor Unbalance Protection
RE: Problem with Double Wye Capacitor Unbalance Protection
I never questioned it before, but is the 13.8kV system a grounded Y system? If so, you might be able to co-ordinate the 59N relay trip time with the other relays. However, if it's not a grounded Y system then that 59N relay connection will likely always be troublesome.
RE: Problem with Double Wye Capacitor Unbalance Protection
busbar: yeas (blown fuse = open fuse)
Stevenal: i like the idea of delaying the 59N some time till the faulty feeder protection operate,
is it usual practice?
LionelHutz: shorted capacitor section will not cause voltage stress on other unit. I think we are taking about open fuse.
thank you
RE: Problem with Double Wye Capacitor Unbalance Protection
RE: Problem with Double Wye Capacitor Unbalance Protection
My primary concern when I built the double Y banks was to protect against shorted capacitor elements causing an over-voltage stress on the other capacitor elements.
I wasn't nearly as concerned with blown fuse detection, mostly because a blown fuse would not cause a capacitor can to explode. I considered the odds of either 2 capacitor elements or 2 fuses on the same phase failing at exactly the same time to be fairly slim enough I was not protecting against those cases.
I still believe one of those multi PT scheme's is the proper way to protect the bank against blown fuses. If you don't want to apply such a scheme then you will have to adjust the settings until you get the results you want, regardless of "usual practice".
Maybe you should consider a scheme such as Fig 11. Take a look at a Cooper Power iCP-440 relay.
RE: Problem with Double Wye Capacitor Unbalance Protection
If using a fuseless capacitor, the protection philosophy shifts to detecting the short(s).
Both conditions cause overvoltage. Good equations for your spreadsheet can be found in C37.99.
RE: Problem with Double Wye Capacitor Unbalance Protection
please see
RE: Problem with Double Wye Capacitor Unbalance Protection
Meant C37.99.